CS212: OPERATING SYSTEM

Lecture 6: Virtual-Memory Management

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Computer Science

Department

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Chapter 9: Virtual-Memory Management

Background Demand Paging Page Replacement Allocation of Frames Thrashing

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Objectives

To describe the benefits of a virtual memory system

To explain the concepts of demand paging, page-replacement algorithms, and allocation of page frames

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Background

Virtual memory – separation of user logical memory from physical memory. Only part of the program needs to be in memory for

execution Logical address space can therefore be much larger than

physical address space Allows address spaces to be shared by several

processes Allows for more efficient process creation

Virtual memory can be implemented via: Demand paging Demand segmentation

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Virtual Memory That is Larger Than Physical Memory

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Virtual-address Space

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Shared Library Using Virtual Memory

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Demand Paging

Bring a page into memory only when it is needed Less I/O needed Less memory needed Faster response More users

Page is needed reference to it invalid reference abort not-in-memory bring to memory

Lazy swapper – never swaps a page into memory unless page will be needed Swapper that deals with pages is a pager

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Transfer of a Paged Memory to Contiguous Disk Space

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Valid-Invalid Bit With each page table entry a valid–invalid bit is associated

(v in-memory, i not-in-memory) Initially valid–invalid bit is set to i on all entries Example of a page table snapshot:

During address translation, if valid–invalid bit in page table entry

is I page fault

vvv

v

i

ii

….

Frame # valid-invalid bit

page table

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Page Table When Some Pages Are Not in Main Memory

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Page Fault

If there is a reference to a page, first reference to that page will trap to operating system:

page fault1. Operating system looks at another table to

decide: Invalid reference abort Just not in memory

2. Get empty frame3. Swap page into frame4. Reset tables5. Set validation bit = v6. Restart the instruction that caused the page fault

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Steps in Handling a Page Fault

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Page Replacement

What happens if there is no free frame?

The solution:

Page replacement – find some page in memory, but not really in use, swap it out algorithm performance – want an algorithm which

will result in minimum number of page faults

Same page may be brought into memory several times

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Need For Page Replacement

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Basic Page Replacement

1. Find the location of the desired page on disk

2. Find a free frame: - If there is a free frame, use it - If there is no free frame, use a page replacement algorithm to select a victim frame

3. Bring the desired page into the (newly) free frame; update the page and frame tables

4. Restart the process

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Page Replacement

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Thrashing If a process does not have “enough”

frames, the page-fault rate is very high. This leads to: low CPU utilization operating system thinks that it needs to

increase the degree of multiprogramming another process added to the system

Thrashing a process is busy swapping pages in and out

A process is thrashing if it is spending more time paging than executing.